1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic method.
2. Description of the Related Art
In an image forming apparatus using an electrophotographic method, a scanner unit irradiates a photosensitive member with light to form an electrostatic latent image thereon. After the formed electrostatic latent image is developed with toner, the developed image is transferred onto the recording medium. In this way, an image is formed on a recording medium. With this type of apparatus, if images are formed continuously over a long time, the temperature inside the apparatus is increased. Consequently, image misregistration may be caused. Such image misregistration signifies shifting of the light, which is radiated from the scanner unit to the photosensitive member, from an appropriate irradiation position. A primary cause of the image misregistration is a change of characteristics of a lens or the like included in the scanner unit by an increase of the temperature. In addition, other causes of the image misregistration include mechanical installation errors of the photosensitive member and the scanner unit and eccentricity of the photosensitive member or a gear for driving the photosensitive member. Various measures for correcting the image misregistration caused by these reasons have been demanded.
In addition, if a single photosensitive member is used as described above, misregistration of an image formation position is simply caused. However, in the case of a color image forming apparatus that forms an image by using a plurality of photosensitive members, the image misregistration results in color misregistration. Namely, for example, when different colors of images formed on the respective photosensitive members are sequentially superimposed and transferred onto an intermediate transfer member, if the transfer position of an image of a certain color is misregistered relative to the transfer positions of the images of the other colors, color misregistration is caused in the superimposed color image.
The image misregistration can be caused in a sub-scanning direction, which is the direction in which the photosensitive member is rotated, and in a main-scanning direction, which is the direction in which the light radiated from the scanner unit scans the photosensitive member. For example, if the image misregistration in the main-scanning direction is caused, the image is tilted in the main-scanning direction or the length of the image is changed in the main-scanning direction.
For example, Japanese Patent Application Laid-Open No. 7-234612 discusses a method for correcting the image misregistration. According to this method, each color of toner pattern is transferred from a corresponding one of a plurality of photosensitive members onto a transfer belt, relative positions of these toner patterns are detected by using optical sensors, and the image misregistration in the sub- and main-scanning directions is corrected based on the detection results. However, since the method discussed in Japanese Patent Application Laid-Open No. 7-234612 requires cleaning of the toner patterns formed on the transfer belt, down time is caused. Recently, for example, Japanese Patent Application Laid-Open No. 2012-032777 discusses an image forming apparatus capable of reducing such down time caused when the image misregistration is corrected. This image forming apparatus detects the image misregistration in the sub-scanning direction by using an electrostatic latent image pattern formed on a photosensitive member and corrects the image misregistration by using the detection results. When correcting the image misregistration in the sub-scanning direction, the image forming apparatus discussed in Japanese Patent Application Laid-Open No. 2012-032777 uses electrostatic latent image patterns without using toner patterns. Thus, an operation of cleaning toner patterns is not necessary. Therefore, the image forming apparatus is excellent in terms of reduction in down time and in reduction in consumption of toner.
However, as described above, the image misregistration may be caused not only in the sub-scanning direction but also in the main-scanning direction. If the image misregistration in the main-scanning direction is corrected by using toner patterns in a conventional manner, down time is caused.